Anybody knows how MySQL allocates disk space for fields like "TEXT" or "BLOB"
For example, what happens when I insert 10kb string into "TEXT" column? Is the entire 65kb data allocated or only 10kb?
This is explained in the documentation: http://dev.mysql.com/doc/refman/5.7/en/storage-requirements.html
BLOB, TEXT L + 2 bytes, where L < 2^16 MEDIUMBLOB, MEDIUMTEXT L + 3 bytes, where L < 2^24 LONGBLOB, LONGTEXT L + 4 bytes, where L < 2^32
Variable-length string types are stored using a length prefix plus data. The length prefix requires from one to four bytes depending on the data type, and the value of the prefix is L (the byte length of the string). For example, storage for a MEDIUMTEXT value requires L bytes to store the value plus three bytes to store the length of the value.
So in short, the whole 65kb is not wasted.